ixgbe: DCB, set minimum bandwidth per traffic class

DCB provides a guaranteed bandwidth in the case with 0%
bandwidth then no bandwidth is guaranteed. However the
traffic class should still be able to transmit traffic.
For this to work the traffic class must be given the
minimum credits required to send a frame.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>